"I don't even agree with my mother 98 percent of the time, much less the president of the United States," McCaskill countered.

It has become entirely common this year for Republicans to try to tar their opponents by association with Obama. "In those battleground states that have a Senate race, the numbers in the Senate race, like a draft in a bike race, have followed Romney's numbers," says GOP pollster Ed Goeas.

It's happening all the way down the ballot. In Missouri, the Republican candidate for attorney general, Ed Martin, commonly refers to Democratic incumbent Chris Koster as "Obama's lawyer." The back of Martin's yard signs, in fact, say, "Fire Obama's Lawyer."

Conversely, Romney is turning out to be a drag on certain GOP candidates — notably Republican Sen. Scott Brown in Romney's political home state of Massachusetts.

Brown, who has been hurt by Obama's big polling lead in the state, has sought at times to distance himself from Romney and other national Republicans, saying for instance that Romney's comments about the "47 percent" of Americans who depend on government are not the way he views the world.

Linda McMahon, the Republican Senate nominee in heavily Democratic Connecticut, began airing an ad this past weekend showcasing voters who were going to split their ticket, supporting both her and Obama.

But it's not the navy blue and scarlet red states where coattails will have the most effect, Goeas says.

It's in states and districts that are most competitive where the presidential campaigns matter most down ballot. That's because of the partisan interest they generate and the effort and resources they marshal to turn out voters.

"The reason is simple: There are fewer and fewer split-ticket voters in this highly partisan age," says Whit Ayres, a Republican consultant. "There are simply not that many people who are going to vote for Romney and a Democrat for Senate."

Democrats in swing states such as Colorado, Nevada and Iowa are looking to Obama's turnout machine to help them maintain control of contested legislative chambers. "We need to win the top of the ticket to win the bottom of the ticket," says Matt McCoy, a Democratic state senator in Iowa. "We're totally tied together. That's why we're embracing the president."

Plenty of candidates for Congress or state legislature will say the effect of coattails is ultimately minimal, that they're running their own races and establishing their own identities and messages.

But most will admit that a strong partisan wave threatens to wipe them away. That's why candidates are making calls or knocking on doors seven nights a week — or six, in parts of the country where campaigning on Sundays is still frowned upon.

"When it comes to getting your message out, it comes down to having the resources and also making the effort," says Ben Lange, a GOP candidate for Congress in Iowa.

Candidates like Lange believe they can run just a bit ahead of their ticket by showing up at parades, knocking on doors and making a personal impression.

"You always get drowned out down ticket, with the amount of money Obama and Romney are spending hammering at each other," says Matt Reisetter, a Republican candidate for state Senate in Iowa. "You're out here working your butt off because you're trying to get over that 3 to 4 percent hump."